Add AGENTS.md to .fernignore to preserve it during SDK regeneration

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: kraenhansen

.fernignore

@@ -15,6 +15,7 @@ src/elevenlabs/realtime/
 .github/
 
 README.md
+AGENTS.md
 assets/
 
 # Ignore custom tests